Users of obscure third-party Twitter applications may be surprised to find that their apps no longer work, if the app creators of those apps haven't been keeping up with changes in the Twitter API (application programming interface).

Microblogging service Twitter is in the final stages of migrating its sign-on service for third party applications to a different of authentication protocol, called OAuth.

Users logging through the Twitter Web site will not notice the difference, nor should users of third-party apps that have already made the switchover, including many popular ones such as TweetDeck, Twitterrific, Seesmic, and Twitter for Android. But if the app hasn't been updated in a while, and still requires a Twitter user name and password, then it will probably stop working correctly.

Over the past month, Twitter has periodically lowered the number of data requests that apps could make to Twitter each hour, as a way of weaning third-party application developers from the old authentication procedure, called Basic Auth. As of 8 AM Pacific time, Tuesday, Twitter will reject any requests from third-party applications that use Basic Auth.

"Basic Auth for Twitter is almost history. Rate limits are down to 15 requests/hour, and will be 0 by tomorrow," wrote Twitter creative director Doug Bowman in a short post on the Twitter site Monday. The rate limit for OAuth is 350 requests per hour.

Twitter announced the switchover last December, though it pushed back its deadline for the switch from the end of June to the end of August.

On a page explaining the reasons behind the change, Twitter gave several reasons that OAuth is superior to Basic Auth. The new protocol won't ask users to provide the password directly to third-party sites. It makes spoofing of applications more difficult. It will help Twitter fight spam, and it paves the way for more trusted services.

When a user signs onto a third-party application with OAuth, the app itself doesn't get access to the user name and password. Instead, Twitter itself will provide a sign-in module, which in turn provides a key to the application provider should the log-in succeed.

With this approach, users don't have to supply passwords to each third-party application, and won't have to manage multiple passwords for multiple services (though the Twitter help page on this topic notes that some third-party services, such as desktop applications or mobile apps for submitting messages will still require passwords).

To what extent third-party application builders have gotten the message about OAuth is unclear. As of earlier this week, some were still surprised at the lowered rate limit.

Others, however, got an early start.

"Despite a few early hurdles, I'm very glad I made the switch to OAuth early," said Dean Robinson, creator of the Hahlo mobile Twitter client. Robinson took the requirement as an opportunity to do a major revision of his software, fixing a number of other issues as well. Those using version 3 of Hahlo will now have to upgrade to the new version, version 4. He noted that the documentation that Twitter supplied was fairly straightforward.

"The support and knowledge about OAuth, in particular in the official [development] group, is significantly better than ... when I was trying to implement the changes. I'd be really surprised if anyone has had major issues in making the transition recently," Robinson said.